1. Field of the Invention
The present invention relates to a flat group-delay low-pass filter which prevents and minimizes degradation in a signal waveform with flat group-delay characteristics and attenuates out-of-band noise, and also relates to an optical signal receiver including such a flat group-delay low-pass filter.
2. Description of the Related Art
A Bessel low-pass filter can be used to remove a noise component and to improve an error rate in digital communications using an optical fiber.
The Bessel low-pass filter is a ladder-type circuit including an inductor defining a series element and a capacitor defining a shunt element with the one end thereof grounded. To provide flat group-delay characteristics, the argument arg T of the transfer function T(jxcfx89) of the filter is approximately proportional to the frequency. If the number of stages of the filter is determined, the values of circuit elements are automatically determined. The value of elements are easily calculated using normalization parameters of a standard low-pass filter, such as those described in xe2x80x9cHandbook of FILTER SYNTHESISxe2x80x9d authored by A. I. Zverev.
FIG. 19 shows a circuit arrangement of a four-stage Bessel low-pass filter. FIGS. 20A and 20B show transmission characteristics, reflective characteristics, and group-delay characteristics. Referring to FIG. 19, L1 and L2 are inductors defining series elements, and C1 and C2 are capacitors defining shunt elements. As shown in FIG. 20A, S11 represents reflective characteristics, and S21 represents transmission characteristics. In this example, the cut-off frequency (at which a signal is attenuated by 3 dB) is 7.5 GHz. Referring to FIG. 20B, the group-delay characteristic is flat, and distortion of signals in a wide band is controlled, and harmonic noise is removed.
The filter illustrated in FIG. 19 has poor reflective characteristics, and suffers from undulation in transmission characteristics due to multiple reflections as a result of mismatching between prior and subsequent stages. To improve reflective characteristics, fixed attenuators are arranged at the front end and back end of the filter.
A fixed attenuator, if inserted for impedance matching, not only causes a transmission loss but also is not economical.
To improve reflective characteristics, Japanese Unexamined Patent Application Publication No. 9-270655 discloses a correction circuit which includes a series circuit having a capacitor and a parallel circuit having an inductor and a resistor, and is used as a shunt element for a Bessel low-pass filter. FIG. 21 illustrates the internal construction of the filter in the above-described publication. Referring to FIG. 21, R1-R4 are resistors, C1-C4 are capacitors, and L1xe2x80x2-L4xe2x80x2 are inductors. L1-L4 are inductors constructed of metal ribbons. Support insulators 5, 6, 7, and 8 are bonded to a metal plate 4 for supporting the inductors L1, L2, and L3 defining series elements.
FIG. 22 is a circuit diagram of the filter shown in FIG. 21. FIG. 22 shows only four stages of the filter. Referring to FIG. 22, Lw is an inductance of a wire connecting each of resistors R1 and R2 to another element.
In the filter illustrated in FIG. 21, the inductance varies depending on variations in the length of each wire connecting each of the resistors R1-R4 to other elements, and the group-delay characteristics of the filter change significantly. For this reason, the characteristics need to be adjusted, thereby increasing the manufacturing cost of the filter. Furthermore, this arrangement must use an inductor, having an inductance that is larger than that of the series element, such as a spiral inductor defining the shunt element. Such an inductor is expensive compared with the capacitor or the resistor, thereby increasing the cost of the filter.
In order to overcome the problems described above, preferred embodiments of the present invention provide a flat group-delay low-pass filter which eliminates the need for arranging fixed attenuators at the front end and the back end of the filter with the attenuators for controlling the effect of reflections caused by impedance mismatching with components arranged in front of and in back of the filter, and which is low in manufacturing costs and component costs, and to provide an optical signal receiver including such a novel flat group-delay low-pass filter.
Preferred embodiments of the present invention also provide a flat group-delay low-pass filter including a series circuit having a capacitor and a resistor with one end of the series circuit grounded, defining a shunt element, and an inductor defining a series element.
In this arrangement, the resistance component of the series circuit has a frequency characteristic such that the flat group-delay low-pass filter has an ideally flat group-delay characteristic in a low frequency range while a signal is absorbed by the resistor in a high frequency range.
Preferably, each of the inductor and the capacitor includes an electrode pattern on the dielectric substrate, and the resistor includes a resistive film pattern on the dielectric substrate. This arrangement eliminates variations in characteristics due to variations in wires, thereby making the adjustment of the characteristics of the filter unnecessary.
Preferably, the inductor includes a micro strip line having a ground electrode arranged on the bottom surface of a dielectric substrate and a line electrode pattern disposed on the top surface of the dielectric substrate, and the line electrode pattern of the micro strip line is constructed such that the characteristic impedance of the micro strip line is larger than about 50 xcexa9, and an electrode for external connection connected to the electrode pattern of the micro strip line is disposed on the top surface of the dielectric substrate.
Preferably, the line electrode pattern is manufactured using a thin-film formation process with the outermost surface thereof being formed of Au, and the resistive film pattern is preferably formed of a tantalum oxide film using a thin-film formation process.
Preferably, the flat group-delay low-pass filter further includes a ground electrode pattern connected to the ground electrode and arranged on at least one of the two side regions of the external connection electrode.
Preferably, the flat group-delay low-pass filter further includes a protective layer covering the inductor, the capacitors and the resistor on the dielectric substrate.
Preferably, the external connection electrode is connected to an external circuit through a wire.
Preferably, the flat group-delay low-pass filter further includes a side electrode extending from the external connection electrode and arranged on the side wall of the dielectric substrate.
Preferably, the flat group-delay low-pass filter further includes a bump, disposed on at least the external connection electrode on the top surface of the dielectric substrate, for flip-chip bonding.
Preferably, a flat group-delay low-pass filter device of preferred embodiments of the present invention includes one of the above-referenced flat group-delay low-pass filters and a package base on which the flat group-delay low-pass filter is mounted, wherein the package base has a thick-film terminal electrode for surface-mounting, whereby the package base is surface-mounted on a board of an electronic apparatus such as an optical signal receiver.
An optical signal receiver of a preferred embodiment of the present invention includes a photoelectric converter circuit for converting a received optical signal into an electrical signal, and the flat group-delay low-pass filter that receives an electrical signal into which the photoelectric converter circuit converts the optical signal, and attenuates a high-frequency component of the electrical signal. In digital communications using an optical fiber, out-of-band noise is reduced without degrading the waveform of the signal.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.